Foreign development regarding devices for speed regulating of in-pipe robot inspection has been commercially applied in gas pipeline this decade, however, the results of theoretical and structural research on bypass device were scaled in and abroad after the research of literature. First, this paper deals with the new structure of a butterfly bypass-valve, the basic parts and working principle on this speed regulating unit. The using of a butterfly disc in bypass-hole of PIG (pipeline inspection gauge) can achieve a speed regulating purpose in bypass-PIG. Next, the theoretical model of pressure loss and regulating feasibility or ability of PIG are introduced by the similar mathematical model with a throttle valve. Finally, give the conclusion.